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• Engine brakes and retarders used to be seen as

11th May 2000, Page 34
11th May 2000
Page 34
Page 35
Page 34, 11th May 2000 — • Engine brakes and retarders used to be seen as
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special equipment purely for Hazchern and heavy haulage operators, but for many operators it is no longer a question of whether to fit a supplementary braking system, but simply deciding which type.

Service brakes have to work harder because gross weights have gone up and smaller wheels have reduced the available friction area. Drivelines have become more efficient; there is reduced aerody

namic drag and rolling resistance on tyres. Irregular traffic flows demand more frequent braking.

Engine brakes and retarders relieve the service brakes of much of their work, so they remain cool and in peak condition for emergencies. But the bottom-line is that brake maintenance costs can be dramatically reduced --by up to 8o% on some operations, according to ZE and Telma.

Integrated braking systems are the latest trend. Electronic con

trols allow all the braking functions to be incorporated into one unit, reducing reliance on the service brakes. Braking is first by the engine brake, then the retarder, and finally by the service brakes.

Engine brakes

Engine brakes deliver more retardation than exhaust brakes; they are offered by most manufacturers on engines over io litres. They are cheaper than a retarder, but perform best in high gears and at high engine revs. In effect an engine brake converts a diesel engine into an air compressor via an exhaust cut-out valve or a constantly open throttle valve. The larger the engine displacement, the more powerful the compressor. Retardation is also affected by compression ratio, turbo boost and valve-opening timing.

Air is compressed by the crankshaft The higher the compression ratio, the higher the cylinder pressures, and the greater the power produced. A turbocharger can boost this further.

Volvo has developed its own exhaust-valve brake for its 12-litre engine, but many manufacturers use a variant of the Jake engine brake, developed originally by US manufacturer Jacobs for Cummins engines. The Daf Engine Brake, for example, which is fitted on the 12.6-litre engine, is effectively a Jake model.

In normal mode, when no fuel is being injected, the exhaust valves remain closed at the top of the compression stroke. The power generated by air pressure on the piston is returned to the crankshaft. When the engine brake is activated the exhaust valves are opened, no power is returned to the crankshaft and the engine provides retarding power.

Timing of the valve opening is crucial for maximum efficiency A cam or rocker motion ensures valve opening occurs close to top dead-centre. The motion is picked up by a master piston and transmitted through a hydraulic circuit to a slave piston which opens the exhaust valve. This releases compressed air from the cylinder to the atmosphere via the manifold, completing a compression-braking circuit. A cut-off switch prevents its use below goorpm to prevent the engine stalling.

The constant-throttle engine brake preferred by MercedesBenz works on a similar principle. However, in this case there is no hydraulic circuit—all the mechanism is buried in the cylinder head, saving weight and noise. Mercedes-Benz says advantages include weight and simplicity: the Actros constant-throttle brake adds just 2kg to the weight.

Retarders

There are two types of retarder: electromagnetic and hydrodynamic. Both are particularly efficient at maintaining a constant speed on downhill gradients.

Direct-coupled retarders are now usually side-mounted on the transmission housing. This eliminates any need to alter propshaft lengths or restrict use of power Lake-off units or emergency auxiliary steering pumps.

Hydrodynamic retarders work by using the fluid flow rate to produce energy In a hydrodynamic retarder a rotor and a stator wheel oppose each other, separated by oil. As the oil flows between them it acts on the rotor with a damping effect that opposes its movement; this in turn brakes the retarder shaft.

The volume and pressure of oil determine the amount of braking torque available. This is also dependent on propshaft speed and the ability of the cooling circuit to dissipate heat. A hydrodynamic retarder is not effective at speeds below 20krn/h.

ZF and Voith are the market leaders in Europe for hydrodynamic retarders. Both companies supply European truck manufacturers, Tracks fitted with ZF transmissions, including the Iveco and MAN ranges, have optional Intarders.

The Voith 115H retarder is designed to adapt to manufacturers' own gearboxes. Renault fits the 115 retarders on Premium trucks, for example, while Volvo fits them on FM and FL models. Scania has developed its own retarder which works on hydrodynamic principles.

These type of retarders weigh in at around 70kg and are generally a third lighter than electromagnetic retarders.

A key difference between ZF and Voith retarders is in the oil circuit ZF Intarders are designed to be integrated with its Ecoseries and AS Tronic transmissions and share the same oil circuit. ZF claims transmissions with Intarders run up to fo'C cooler than

those without, thanks to a greater oil quantity and enhanced cooling via the engine cooling system. Reduced wear and longer oil-change intervals are two of the claimed benefits.

Voith prefers a separate oil circuit to eliminate the risk of heat generated by the retarder causing stress on the transmission. In some applications, says Voith, improved retardation can result because the oil can reach its maximum operating temperature and is not restricted by the capacity of the engine cooling system.

Electromagnetic retarders work on the induced current principle, also railed eddy currents. The braking is generated by energising the vehicle's electrical system to power a series of coils on the fixed stator wheel. Each coil has a pole shoe to distribute the magnetic field created. Air in the gap between the stator and the twin rotors cools the rotors, so unlike a hydrodynamic retarder there is no connection with the engine cooling system.

Electromagnetic retarders have carved out sales in certain niche markets. Unlike hydrodynamic retarders, they perform efficiently at vehicle speeds below 20kr/Vh. That makes them popular for very low-speed operations such as refuse collection and heavy haulage. Electromagnetic retarders are the most popular solution for light diesels on 3.5-tonne chassis, especially among the emergency services. The extra weight of these small retarders is under 6okg, says Telma, the French company which is the UK's leading supplier of electromagnetic retarders.

Telma sells two ranges: Axial and Focal. The Axial range is not specific to any engine or gearbox and can be fitted to any suitable point on the chassis between the gearbox and rear axle. The Focal range, which accounts for 8o% of Telma UK sales, fits directly to the propshaft.

Controls

Retarders usually have two operating modes: continuous braking torque or intermittent operation, in which a pre-selected road speed is maintained when travelling downhill. In continuous mode several levels of torque are available; control is via a steeringcolumn or dashboard stalk.

All manufacturers offer optional foot controls and integration with the throttle and footbrake. In this last case every application of the service brakes engages the retarder. In normal operations, the first three stages are engaged before the service brakes are actuated.

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